In recent years, infrared thermometers have come into wide use for detection of temperature of adults. For core temperature readings, infrared thermometers which are adapted to be inserted into the patient""s ear have been extremely successful. Early infrared thermometers were adapted to extend into the ear canal in order to view the tympanic membrane and provide an uncorrected, direct reading of tympanic temperature which correlates with pulmonary artery temperature. More recently, however, to provide for greater comfort and ease of use, ear thermometers have been designed to provide corrected readings of the generally cooler distal ear canal. Such thermometers measure temperature of distal ear canal tissue and calculate arterial temperature via heat balance.
The arterial heat balance approach is based on a model of heat flow through series thermal resistances from the arterial core temperature to the ear skin temperature and from the ear skin temperature to ambient temperature. Accordingly, after sensing both the skin temperature and ambient temperature, the arterial core temperature can be calculated. The thermal resistance model also allows for computation of equivalent oral and rectal temperatures with the mere adjustment of a weighting factor in the computation.
Infrared ear thermometry has not found such high acceptance for use with neonates. Neonates have a very high moisture level in their ear canals, due to the presence of vernix and residual amniotic fluid, resulting in low ear temperatures because of the associative evaporative cooling. In addition, environmental uncertainties, such as radiant heaters and warming pads can significantly influence the air temperature. Further, clinicians are less inclined to position the tip of an infrared thermometer in the ear of a small neonate.
Infrared thermometers designed for axillary temperature measurements are presented in U.S. patent applications Ser. No. 08/469,484 (now U.S. Pat. No. 5,893,833) and Ser. No. 08/738,300 (now U.S. Pat. No. 5,874,736) which are incorporated herein by reference in their entirety. In each of those devices, an infrared detector probe extends from a temperature display housing and may easily slide into the axilla to lightly touch the apex of the axilla and provide an accurate infrared temperature reading in as little as one-half second. The axillary thermometer also relies on the arterial heat balance approach to provide arterial, oral or rectal temperature.
The axillary infrared thermometer has found great utility not only with neonates but as a screening tool in general, and especially for small children where conventional temperature measurements such as a thermometer under the tongue or a rectal thermometer are difficult.
Unfortunately, the accuracy and repeatability of axillary infrared thermometry with neonates has not extended to older patients. The present invention relates to improvements to the axillary infrared thermometer based on clinical research to improve the device for older patients.
In neonates perfusion is high, and in both the ear of adults and the axilla of neonates, perfusion rates are relatively constant since vasomotor functions are minimal. However, with older patients perfusion rates in the axilla are more variable.
In ear and neonate axillary thermometry, the difference between skin temperature and ambient temperature has been weighted by a coefficient approximating h/pc, where h is an empirically determined coefficient which includes a radiation view factor between the skin tissue and ambient, p is perfusion rate and c is blood specific heat. In ear and neonate axillary thermometry, that coefficient was found empirically to be about 0.09 and 0.05, respectively, with only minor variations. However, with greater exposure for heat transfer and higher vasomotor functions, that coefficient has been determined empirically for the adult axillary region to be about 0.13 with much more significant variations.
Further, it has been determined that perfusion rate varies according to the patient""s temperature. Under febrile (fever) conditions, metabolic demand increases and oxygen consumption increases at a rate greater than that required to sustain a temperature, thereby requiring an increase in perfusion and thus reducing the required weighting coefficient. Under normal, afebrile conditions, normal thermal regulation varies skin temperature over a wider range for skin temperature variation of several degrees with core temperature variation of only a few tenths of a degree.
As in prior ear and axillary thermometers, internal core temperature can be computed from the function
Tc=(1+(h/pc)) (Tsxe2x88x92Ta)+Taxe2x80x83xe2x80x83(1)
where Ts and Ta are the skin and ambient temperatures. The function can be seen to include a weighted difference of surface temperature and ambient temperature with a weighting coefficient h/pc.
In accordance with the present invention, a body (i.e., human or animal) temperature detector comprises a radiation sensor which views a target surface area of the body. Electronics in the detector compute an internal temperature of the body as a function of ambient temperature and sensed surface temperature. In accordance with one aspect of the invention, the function includes a weighted difference of surface temperature and ambient temperature, the weighting being varied with target temperature. In particular, the weighting is an approximation of h/pc where h is a heat transfer coefficient between the target surface and ambient, p is perfusion rate and c is blood specific heat.
In a preferred embodiment, the normal approximation of h/pc is about 0.13 for afebrile (nonfever) conditions, and it varies over a range of at least 0.09 to 0.13. Preferably, the approximation of h/pc varies over a range of at least 20% of normal, and the variation with sensed surface temperature is at least 0.01/xc2x0 F. Preferably the change of h/pc is about xe2x88x920.02/xc2x0 F. at about normal axillary temperature of 97xc2x0 F.
The preferred approximation of h/pc is an expression that approximates the idealized straight line segments representing normal and febrile conditions combined to form a curve with a smooth transition from one physiological regime to the other. The expression may be an exponential, two straight line segments, a single straight line segment or, preferably, a polynomial.
The arterial heat balance approach is based on a steady state model, and the ambient temperature Ta used in the arterial heat balance function has been taken as the sensed detector temperature. However, thermal equalization of a temperature detector with its measurement environment may take many seconds, so the detector may in fact be cooler or warmer than the ambient environment to which the target skin had been exposed prior to the measurement. Though the detector obtains an accurate skin temperature reading with reference to detector temperature, the detector temperature may not serve as an accurate indication of the steady state ambient temperature which led to that skin temperature.
It has been determined empirically that a more appropriate ambient temperature to be used in the heat balance computation is an assumed temperature of about 80xc2x0 F. That assumed temperature can be improved by giving some weight to the sensed detector temperature, though significantly less than the 100% weight given in prior heat balance calculations.
Accordingly, in accordance with another aspect of the invention, where the electronics compute an internal temperature of the body as a function of ambient temperature and sensed surface temperature, the ambient temperature within the function is an assumed ambient temperature, the preferred being about 80xc2x0 F. Preferably, the assumed ambient temperature is modified as a function of sensed detector temperature, a change in assumed ambient temperature relative to change in detector temperature being significantly less than 1. Preferably, that change in assumed ambient temperature relative to change in sensed detector temperature is less than 0.5, and most preferably it is about 0.2. Alternatively, the assumed ambient temperature may be fixed, again preferably at 80xc2x0 F.